1. Field of the Invention
The present invention relates to a roller member used for electrophotography, a process cartridge using the roller member and an electrophotographic image forming apparatus (hereinafter, referred to as an “electrophotographic apparatus”) using the roller member.
2. Description of the Related Art
An electrophotographic apparatus adopting an electrophotographic method is mainly composed of an electrophotographic photosensitive member (hereinafter, may be simply referred to as a “photosensitive member”), a charging device, an exposure device, a developing device, a transfer device and a fixing device. The roller member is suitably used in the charging device, the developing device, the transfer device and the fixing device. In the charging device, the roller member is disposed in contact with or close to the surface of the photosensitive member, and charges the surface of the photosensitive member electrically by the applying of a voltage (a voltage only of a direct-current voltage or a voltage of a direct-current voltage with an alternating-current voltage superposed thereon).
Japanese Patent Application Laid-Open No. 2008-276026 discloses a charging roller having protrusions derived from electroconductive resin particles, as a roller member (hereinafter, referred to as a “charging roller”) for charging a photosensitive member electrically in such a way that the roller member is brought into contact with the photosensitive member.
However, in the charging roller according to Japanese Patent Application Laid-Open No. 2008-276026, when the charging roller comes into contact with the photosensitive member, pressure is concentrated on the protrusions derived from the resin particles, to result in non-uniform abrasion of the surface of the photosensitive member in use over a long term.
For such a problem, Japanese Patent Application Laid-Open No. 2011-237470 discloses a roller member including bowl-shaped resin particles having openings in the electroconductive resin layer thereof and having on the surface thereof a concave-convex shape derived from the openings and edges of the bowl-shaped resin particles. The roller member according to Japanese Patent Application Laid-Open No. 2011-237470 is described to be capable of suppressing the non-uniform abrasion of the photosensitive member even in use over a long term, due to the alleviation of the contact pressure as a result of the elastic deformation at the time when the edges of the bowl-shaped resin particles come into contact with the photosensitive member.
In the roller member according to Japanese Patent Application Laid-Open No. 2011-237470, the elastic deformation of the edges of the openings of the bowl-shaped resin particles alleviates the pressure of contact between the edges and the photosensitive member. For the above-described reason, the non-uniform abrasion of the surface of the photosensitive member is suppressed even in use of the photosensitive member over a long term. On the other hand, in the roller member according to Japanese Patent Application Laid-Open No. 2011-237470, there is a possibility that the performance of rotation of the roller member following-up the rotation of the photosensitive member (hereinafter, may be referred to as “follow-up rotation performance”) is degraded.
Recently, with increase in process speed of electrophotographic apparatuses, there has been found a tendency such that when an electrophotographic image is formed, the photosensitive member involved tends to vibrate. When the photosensitive member in a state of vibration is to be charged by bringing a roller member which is low in the follow-up rotation performance into contact with the photosensitive member, the roller member may not follow-up the rotation of the photosensitive member to consequently cause such a phenomenon (hereinafter, also referred to as “stick-slip”) that the roller member slips on the surface of the photosensitive member. The occurrence of the stick-slip causes charging unevenness on the photosensitive member to cause the occurrence of horizontal-line-like density unevenness on the electrophotographic image. Hereinafter, the horizontal-line-like density unevenness occurring on the electrophotographic image may be referred to as “banding.” Additionally, an electrophotographic image on which the horizontal-line-like density unevenness occurs may be referred to as a “banding image.”